1. Field of the Invention
The present invention relates to a light emitting element driving device and can be applied to a light emitting element driving device which can operate a plurality of LEDs.
2. Description of the Background Art
As a light source for backlight of a liquid crystal display, a light emitting element such as an LED (Light Emitting Diode) is used. For the purpose of controlling visibility and suppressing an increase of the power consumption by lighting, a function of controlling the amount of light (hereinafter, referred to as “dimming function”) is provided.
The LED has a characteristic of changing its emission wavelength in accordance with a current value to be inputted. Further, by performing a PWM (Pulse Wide Modulation) control of the inputted current, the above dimming function is controlled.
Therefore, by controlling the pulse width of the PWM control while keeping the current value supplied to the LED constant, brightness can be changed without changing the emission wavelength of the LED.
FIG. 4 is a block diagram showing a circuit configuration of an LED driving device under the PWM control in the background art.
As shown in FIG. 4, the LED driving device comprises a step-up type DC-DC converter circuit 101, a first switch element 102 and a resistor 103. The step-up type DC-DC converter circuit 101 is constituted of an inductance 104, a second switch element 105, a diode 106, a capacitor 107 and a control circuit 108. To an output portion of the step-up type DC-DC converter circuit 101, a plurality of LEDs 109 are connected in series.
Next, an operation of the LED driving device in the background art will be discussed.
A control signal generation circuit 110 outputs a control signal for turn-on from an OFF state. Then, the first switch element 102 is brought into conduction and the control circuit 108 serves to increase an output voltage Vo of the step-up type DC-DC converter circuit 101.
It is assumed that the output voltage Vo exceeds a total forward voltage Vf of the LEDs 109 connected in series. Then, from the capacitor 107, a current Io flows into the LEDs 109, the first switch element 102 and the resistor 103.
The control circuit 108 monitors a voltage drop Vp of the resistor 103. Further, in the control circuit 108, a reference voltage Vref is set in advance. The control circuit 108 controls the second switch element 105 to be turned off if the voltage drop Vp reaches the reference voltage Vref.
As discussed above, when the second switch element 105 is turned off, the output voltage Vo of the step-up type DC-DC converter circuit 101 decreases and the current Io flowing from the capacitor 107 toward the LEDs 109 and the like also decreases. When the current value Io decreases, the voltage drop Vp of the resistor 103 also decreases. If the voltage drop Vp becomes lower than the reference voltage Vref, the control circuit 108 controls the second switch element 105 to be turned on.
Thus, since turn-on and turn-off of the second switch element 105 are repeated under the control of the control circuit 108, a constant current flows in the LEDs 109.
It is assumed that a control signal for turn-off from the ON state is outputted from the control signal generation circuit 110 for dimming the LEDs 109. Then, the first switch element 102 is interrupted and the current Io flowing in the LEDs 109 and the like thereby sharply decreases. When the current Io decreases, the voltage drop Vp of the resistor 103 becomes lower than the reference voltage Vref and the control circuit 108 serves to increase the output voltage Vo of the step-up type DC-DC converter circuit 101.
In order to prevent an increase of the output voltage Vo, the second switch element 105 is turned off at the same time as the first switch element 102 is turned off. Then, the power supply from the step-up type DC-DC converter circuit 101 to the LEDs 109 is stopped, the voltage across the capacitor 107 is kept almost equal to the total forward voltage Vf of the LEDs 109. An input voltage Vi does not have a value enough to drive the LEDs 109.
Therefore, by turning off both the switch elements 102 and 105 at the same time, in effect, the power supply from the step-up type DC-DC converter circuit 101 to the LEDs 109 is stopped and the voltage across the capacitor 107 is kept almost equal to the total forward voltage Vf of the LEDs 109.
The conventional mainstream of light source for backlight is a power-saving type used for an indicator, a portable device or the like. Therefore, the voltage across the capacitor 107 can be kept by a small capacity.
In recent, however, a high power type light emitting element (such as an LED) which can be used for a light source for backlight of an illumination equipment or a large-scale liquid crystal display has been developed. The power consumed in the light emitting element thereby becomes larger than conventional one. With an increase of the power consumed in the light emitting element, there arises a necessity to increase the capacity of the capacitor 107.
The upsizing of capacity of the capacitor 107 increases an inrush current at turn-on of a main power supply of a light emitting element driving device. FIG. 5 shows generation of the inrush current. In FIG. 5, the upper stage shows an output signal waveform from the control signal generation circuit 110, and the lower stage shows a current waveform in the inductance 104. As discussed above, when the inrush current becomes too large, the circuit portions such as the second switch element 105 and the LED 109 may be broken.